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SAGE-Hindawi Access to ResearchInternational Journal of NephrologyVolume 2011, Article ID 591731, 9 pagesdoi:10.4061/2011/591731

Review Article

Potential Use of Stem Cells for Kidney Regeneration

Takashi Yokoo, Kei Matsumoto, and Shinya Yokote

Project Laboratory for Kidney Regeneration, Institute of DNA Medicine, Department of Internal Medicine,The Jikei University School of Medicine, Tokyo, 105-8461, Japan

Correspondence should be addressed to Takashi Yokoo, [email protected]

Received 11 January 2011; Accepted 18 February 2011

Academic Editor: Niels Olsen Saraiva Camara

Copyright © 2011 Takashi Yokoo et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Significant advances have been made in stem cell research over the past decade. A number of nonhematopoietic sources of stemcells (or progenitor cells) have been identified, including endothelial stem cells and neural stem cells. These discoveries have beena major step toward the use of stem cells for potential clinical applications of organ regeneration. Accordingly, kidney regenerationis currently gaining considerable attention to replace kidney dialysis as the ultimate therapeutic strategy for renal failure. However,due to anatomic complications, the kidney is believed to be the hardest organ to regenerate; it is virtually impossible to imaginesuch a complicated organ being completely rebuilt from pluripotent stem cells by gene or chemical manipulation. Nevertheless,several groups are taking on this big challenge. In this manuscript, current advances in renal stem cell research are reviewed andtheir usefulness for kidney regeneration discussed. We also reviewed the current knowledge of the emerging field of renal stem cellbiology.

1. Introduction

Recent advances in stem cell research have brought the possi-bility of using somatic stem cells for organ regeneration onestep closer to realization. Newly developed organs can thenbe used in clinical organ replacement. However, anatomicallycomplicated organs such as the kidney have proven morerefractory to stem cell-based regenerative techniques.

The kidney retains the potential to regenerate if thedamage is not too severe, and the kidney structure remainsintact.

It was previously believed that bone marrow-derivedstem cells could differentiate into renal-resident cellsand participate in kidney regeneration after renalischemia/reperfusion injury [1, 2]; however, recent studieshave suggested that the number of bone marrow-derivedcells that engraft injured tubules and develop into functionalrenal tissue is very low and, thus, their overall contributionto renal repair would be minor in the setting of acutekidney injury [3, 4]. Also, during the progression of chronicrenal failure, bone marrow-derived stem cells suppliedby intravenous administration located in the kidney andcontributed to attenuate renal fibrosis [5]. However, incases of irreversible damage to the kidney, as can occur

with long-term dialysis, the kidney structure is totally lostand, therefore, these cell therapy approach seems not to beapplicable. Thus, any application of regenerative medicine inend-stage renal disease will require the de novo developmentof an entire functional kidney.

In terms of a functional whole kidney, Chan et al. [6]reported the first attempt to develop a functional wholerenal unit by developing a transplantable pronephros inXenopus. Xenopus presumptive ectoderm, which becomesepidermis and neural tissue in normal development, con-tains pluripotent stem cells and can be differentiated intomultilineage tissue cells under particular culture conditions[7]. The researchers designed conditions for the inductionof pronephric tubule-like structures from animal caps thatinvolved a combination of activin and retinoic acid foronly 3 hours. This pronephros-like tissue was transplantedinto bilaterally nephrectomized tadpoles to test for its func-tional integrity as a pronephros. Bilateral pronephrectomyinduces severe edema in tadpoles owing to their inabilityto excrete internal water, and tadpoles die within 9 days;transplantation of the pronephros-like unit at least partiallycorrected the edema, and tadpoles survived for up to 1month. To our knowledge, this study is the only one to

2 International Journal of Nephrology

develop a transplantable functional whole kidney unit invitro, although the pronephros structure formed was tooprimitive for any clinical application in humans.

Since then, many attempts have been made worldwide toregenerate whole kidneys applicable for mammals from stemcells. The present article reviews the challenges and recentadvances in renal stem cell research and discusses its potentialfor clinical application for kidney regeneration in humans.

2. Embryonic Kidney as a Source ofOrgan Regeneration

Woolf et al. [8] reported that the metanephros may continueto grow if it is transplanted into the renal cortex ofhost mice. The developed transplant contains vascularizedglomeruli and mature proximal tubules and may havethe capacity for glomerular filtration. Collecting duct-likestructures appear to extend from the transplant toward thepapilla of the host. Although there is no direct evidencethat these collecting duct-like structures connect with thehost’s collecting system or that the transplant functions ina manner similar to that of a native kidney, the resultsprovide a rationale for the existence of renal stem cellsin the metanephros from early embryos, which may bea potential source of transplantable regenerated kidney.Potential problems with this system include questions asto the suitability of the renal capsule of dialysis patientsas a transplant site, given the significant disruptions tothis area, including the vasculature, and the fact that spacelimitations beneath the renal capsule may hinder the growthof transplants. These concerns may be overcome by thesystem established by Rogers et al. [9]. These researchers alsoused the metanephros as a source of transplantable artificialkidney but transplanted the graft into a host omentum,which is not confined by a tight organ capsule or disturbedby dialysis. Metanephroi from rat, mouse, and pig wereimplanted into the omenta of the rat or mouse, and thesuccess of transplanting across xenogeneic barriers and theextent of differentiation into a functional nephron wereevaluated. This experiment was based on previous studiesshowing minimal immunogenicity in tissues harvested atearlier gestational stages, including the metanephros [10].In cases of allotransplantation (rat metanephros to ratomentum), transplants assumed a kidney-like shape in situthat was approximately one-third the diameter of the nativekidney. Histologically speaking, the transplants containedwell-differentiated kidney structures. It is important tonote that this transplantation technique can be performedwithout immunosuppression. With xenotransplantation, thepig metanephros grew and differentiated into renal tissuein the rat omentum, showing glomeruli, proximal tubules,and collecting ducts; however, immunosuppressants wererequired because without these agents the transplants dis-appeared soon after transplantation. What is interesting isthat the graft pig metanephros was slightly larger in volume(diameter and weight) as compared to that of a normal ratkidney. Furthermore, the transplanted tissue produced urineand, surprisingly, after intact ureteroureterostomy with the

ureter of the kidney that was removed, anephric rats startedto void and showed a prolonged lifespan [11]. This successholds the promise of a new and practical therapeutic strategyfor kidney disease in which a functional renal unit can beestablished by implanting xenometanephros together withimmunosuppression.

Recently, Osafune et al. reported an in vitro culturesystem in which a single Sall1 highly expressing cell fromthe metanephric mesenchyme forms a 3D kidney structureconsisting of glomeruli and renal tubules [12]. More recently,Unbekandt and Davies reported that single-cell suspensionsdissociated from metanephros can reaggregate to form aorganotypic renal structure if they are treated with aninhibitor of Rho-associated kinase [13]. These systemsare useful for examining mechanisms of renal progenitordifferentiation and suggest the possibility of developing awhole kidney from a single stem cell.

3. Kidney Regeneration UsingArtificial Scaffolding

One possible means of creating an optimal niche forpluripotent stem cells to differentiate into a kidney is toprovide an artificial scaffold upon which the stem cells canfunction. This strategy was used recently by Ott et al. [14]to successfully develop a functional artificial rat heart usinga cadaveric heart as the artificial scaffold. A whole-heartscaffold with intact 3D geometry and vasculature was createdby coronary perfusion with detergents into the cadavericheart. This decellularized heart was then repopulated withneonatal cardiac cells or rat aortic endothelial cells andcultured under physiological conditions to simulate organmaturation [14]. The injected neonatal cardiac cells formeda contractile myocardium, which performed the strokefunction.

Cadaveric scaffolds have also been used to developtransplantable liver and lung using mature hepatocyte andalveolar epithelial cells, respectively [15, 16]. After trans-plantation of the recellularized grafts, they were successfullyfunctioning as a hepatocyte and gas exchanger. Such anapproach is promising for regenerating organs that have asimple architecture. However, pluripotent stem cells shouldbe used if this technique is used for kidney regeneration,because the kidney is composed of multiple cell types,such as tubular epithelial cells and glomerular cells. Forthis purpose, experts should know which stem cells mayselectively differentiate into kidney residential cells. One suchcandidate is embryonic stem (ES) cells.

ES cells are undifferentiated pluripotent stem cellsisolated from the inner cell mass of blastocysts [17]. EScells can differentiate into several cell types of mesodermal,endodermal, and ectodermal lineages, depending on cultureconditions. Because human ES cells can differentiate intokidney structures when injected into immunosuppressedmice [18, 19], studies have focused on identifying the preciseculture conditions that allow the differentiation of ES cellsinto renal cells in vitro. To this end, Schuldiner et al. [20]showed that human ES cells cultured with 8 growth factors,

International Journal of Nephrology 3

including hepatocyte growth factor (HGF) and activin A,differentiated into cells expressing WT-1 and renin [19].More recently, mouse ES cells stably transfected with Wnt4(Wnt4-ES cells) were differentiated into tubular structuresexpressing AQP-2 in the presence of HGF and activin A [21].An ex vivo culture system in which ES cells (or ES-derivedcells) were cultured in the developing metanephros was alsoinvestigated to determine the capacity of ES cells to differ-entiate into kidney cells integrated into the kidney structure[22]. ROSA26 ES cells were stimulated with developmentalsignals in the microenvironment of a developing kidneyfollowing injection into a metanephros cultured in vitro.ES cell-derived, β-galactosidase-positive cells were identifiedin epithelial structures resembling renal tubules with anefficiency approaching 50% [22]. Based on these results,Kim and Dressler [23] attempted to identify the nephrogenicgrowth factors needed to induce differentiation of ES cellsinto renal epithelial cells. When injected into a developingmetanephros, ES cells treated with retinoic acid, activin A,and BMP7 contributed to tubular epithelia with near 100%efficiency [23]. Furthermore, Vigneau et al. [24] showedthat ES cells expressing brachyury, a marker of mesodermspecification, became a renal progenitor population in thepresence of activin A. After injection into a developingmetanephros, these cells might be incorporated into theblastema cells of the nephrogenic zone. In addition, a singleinjection of the same cells into developing live newbornmouse kidneys saw them stably integrated into proximaltubules with normal morphology and polarization for 7months without teratoma formation [24]. Taken together,these data highlight ES cells as a potential source of renalstem cells to be differentiated into renal-resident cells.

Based on this series of studies, Ross and coworkersattempted recently to regenerate an entire kidney using adecellularized cadaveric kidney scaffold [25]. After decel-lularization, murine ES cells were infused via the renalartery and were found to localize to the vasculature andglomeruli, with subsequent migration into the tubules.Immunohistochemical analysis suggested that the infusedcells had differentiated into mature kidney cells. Althoughrenal function is yet to be examined, this approach may beuseful for the production of an entire kidney.

To address the adverse effects of immunosuppressants,Lanza et al. [26] attempted to develop a self-kidney unit toeliminate the immune response problem and, therefore, theneed for immunosuppression. To generate a histocompatiblekidney for artificial organ transplantation, Lanza et al.[26] used a nuclear transplantation technique in whichdermal fibroblasts isolated from adult cow were transferredinto enucleated bovine oocytes and then transferred non-surgically into progestin-synchronized recipients. After 6-7weeks, metanephroi were isolated from embryos, digestedusing collagenase, and expanded until the desired cellnumber was obtained by culture in vitro. These cells werethen seeded onto a specialized polymer tube, followed byimplantation into the same cow from which the cells hadbeen cloned. What is striking is that this renal device seededwith cloned metanephric cells appeared to produce a urine-like liquid, whereas those without cells or those seeded with

allogeneic cells did not. Histological analysis of the explantrevealed a well-developed renal structure composed of orga-nized glomeruli-like, tubule-like, vascular elements that wereclearly distinct from one another but continuous within thestructure. Therefore, these renal tissues appeared integrallyconnected in a unidirectional manner to the reservoirs,resulting in the excretion of urine into the collecting system.Although it is not clear how the cultured cells digestedfrom the metanephros gained polarity and self-assembledinto glomeruli and tubules, this technique successfully usednuclear transplantation for renal regeneration without therisks and long-term effects of immunosuppression.

4. De Novo Kidney Regeneration UsingBlastocyst Complementation

It was previously reported that injection of normal EScells into the blastocysts of recombination-activating gene2 (RAG-2)-deficient mice, which have no mature B orT lymphocytes, generated somatic chimeras with ES cell-derived mature B and T cells [27]. This blastocyst comple-mentation system was recently applied to reconstructing theentire human kidney. Injection of wild-type ES cells intothe blastocysts of Sall1-null mice, which lack kidneys, gen-erated metanephroi composed exclusively of ES cell-deriveddifferentiated cells. This strategy, however, was assessedto be unsuitable for human clinical application, becauseit is quite difficult to generate interspecific chimeras inlivestock. Many groups have sought to generate interspecificchimeras between mouse and rat, achieving success withchimeric preimplantation embryos in vitro but failing withlive chimeric animals [28, 29]. Extraembryonic lineage cellslike trophectoderm, derived from xenogenic embryos, maysuffer from inhibition of implantation on exposure to thehost uterus, suggesting that only cells of preblastocyst origincan contribute to the extraembryonic lineage cells. In thiscontext, the discovery of induced pluripotent stem (iPS) cellsallayed this concern.

iPS cells were produced by Takahashi and Yamanaka[30] from cultured somatic cells by retroviral transfer withOct3/4, Sox2, c-Myc, and Klf4, which are transcription fac-tors associated with pluripotency [31]. Rate-reprogrammediPS cells were subsequently derived by the reactivation ofFbx15 [31], Oct4 [31], or Nanog [32], all of which carry adrug resistance marker inserted into the respective endoge-nous locus by homologous recombination or a transgenecontaining the Nanog promoter. Furthermore, iPS cells werealso isolated based on their ES-like morphology, withoutthe use of transgenic donor cells [33], and the therapeuticpotential of autologous iPS cells in a mouse model of ahereditary disease has been reported [34]. Recently, humaniPS cells were successfully induced from adult skin fibroblastsusing the same 4 factors as detailed above [35]. iPS cellsare epigenetically and biologically indistinguishable fromnormal ES cells, and, therefore, it is suggested that theybe used to produce interspecific chimeras. Kobayashi andcoworkers recently reported successful regeneration of ratpancreas in mouse via an interspecific blastocyst injection


of iPS cells [36]. They injected rat iPS cells into Pdx-1−/−

(pancreatogenesis-disabled) mouse blastocysts and foundthat the newborn chimera of rat and mouse processed almostentirely iPS-derived pancreas. This success proves that whenan empty developmental niche for an organ is provided(as with the Pdx1−/− mouse and the pancreatic niche), iPScell-derived cellular progeny can occupy that niche and cancompensate developmentally for the missing contents ofthe niche, forming a complicated organ composed almostentirely of cells derived from donor iPS cells even if theblastocyte complementation involves different species.

Furthermore, instead of tetraploid complementation,Espejel and coworkers generated chimeric mice thatexperienced progressive liver repopulation with iPS cell-derived hepatocytes postnatally using blastocytes deficientin fumarylacetoacetate hydrolase [37]. The entire liver wascomposed of iPS cell-derived hepatocytes by the timethe mice reached adulthood. They replicated the uniqueproliferative capabilities of normal hepatocytes and wereable to regenerate liver after transplantation and two-thirdspartial hepatectomy. These successes strongly suggest thatblastocyst complementation is one of the most promisingstrategies for regenerating the kidney. However, these systemsare not available for clinical use at this time, because it isnot possible to generate the vascular and nervous systems.In addition, the more important ethical issues involved withmanipulating blastocysts with iPS cells remain unresolved.Nonetheless, this success highlights the rationale that theeventual clinical application of kidney regeneration mustdepend on developmental programming.

5. De Novo Kidney Regeneration UsingGrowing Xenoembryos

We attempted to develop an entire kidney using scaffoldingfrom mesenchymal stem cells (MSCs). It has been shownthat MSCs have the capacity for site-specific differentiationinto various cell types of mesodermal lineage, includingchondrocytes, adipocytes, myocytes, cardiomyocytes, bonemarrow stromal cells, and thymic stromal cells [38, 39];this suggests that they may also differentiate into kidneyresidential cells.

Initially, we tried to reconstruct an organized andfunctional kidney structure using a developing heterozoicembryo as an “organ factory.” During embryogenesis, a singlefertilized cell develops into a whole body within 266 (forhumans) or 20 days (for rodents). This neonate has everyorgan positioned correctly, indicating that a single fertilizedovum contains a blueprint from which the body, includingthe kidney, can be built. Therefore, we sought to “borrow”this programming of a developing embryo by applying thestem cells at the niche of organogenesis.

During development of the metanephros, the metaneph-ric mesenchyme initially forms from the caudal portionof the nephrogenic cord [40] and secretes glial cell line-derived neurotrophic factor (GDNF), which induces thenearby Wolffian duct to produce a ureteric bud [41].The metanephric mesenchyme consequently forms the

glomerulus, proximal tubule, loop of Henle, and distaltubule, as well as the interstitium, as a result of reciprocalepithelial-mesenchymal induction between the ureteric budand metanephric mesenchyme [42]. For this epithelial-mesenchymal induction to occur, GDNF must interact withits receptor, c-ret, which is expressed in the Wolffian duct[42]. We hypothesized that GDNF-expressing MSCs maydifferentiate into kidney structures if positioned at thebudding site and stimulated by numerous factors spatiallyand temporally identical to those found in the developmentalmilieu.

To investigate this hypothesis, we initially injectedhuman mesenchymal stem cells (hMSCs) into the developingmetanephros in vitro, although this was not sufficient toachieve kidney organogenesis [43]. This suggests that hMSCsmust be placed before the metanephros begins to develop ina specific, defined embryonic niche to allow their exposureto the repertoire of nephrogenic signals required to generatethe organ. This can be best achieved by implanting hMSCsinto the nephrogenic site of a developing embryo. Therefore,we established a culture system in combination with a whole-embryo culture system, followed by metanephric organ cul-ture (Figure 1). This “relay culture” allows the developmentof the metanephros from structures present before buddinguntil the occurrence of complete organogenesis ex utero.In this system, embryos were isolated from the motherbefore budding and were grown in a culture bottle untilthe formation of a rudimentary kidney so that it could befurther developed by organ culture in vitro [43]. Using thiscombination, rudimentary kidneys continued to grow invitro, as assessed by the observation of fine tubulogenesisand ureteric bud branching, indicating that the metanephroscan finish developing ex utero even if the embryo is dissectedprior to sprouting of the ureteric bud.

Based on these results, hMSCs were microinjectedat the site of budding and subjected to relay culture.Before injection, the hMSCs were genetically engineeredto express GDNF temporally using adenovirus and werelabeled with the LacZ gene and dioctadecyl-3,3,3

′, 3

′-

tetramethylindocarbocyanine. Viral free manipulation canalso be performed using thermoreversible GDNF polymer[44]. Soon after injection, the embryos, together withthe placenta, were transferred to the incubator for wholeembryos. After the relay culture, X-gal-positive cells werescattered throughout the rudimentary metanephros andwere morphologically identical to tubular epithelial cells,interstitial cells, and glomerular epithelial cells [43]. Inaddition, reverse transcription-polymerase chain reactionrevealed the expression of several podocyte- and tubule-specific genes [43]. These data demonstrated that usinga xenobiotic developmental process for growing embryosallows endogenous hMSCs to undergo an epithelial con-version and be transformed into an orchestrated nephronconsisting of glomerular epithelial cells (podocytes) andtubular epithelial cells that are linked. hMSCs can alsodifferentiate into renal stroma after renal development [43].

We then examined the next issue in the successful devel-opment of an artificial kidney de novo: urine production.This requires that the kidney formed have the vascular


∗

(a) (b)

E11.5E12 E12.5 E13

E13.5

E11.5 + 24 h E11.5 + 48 h

(c)

Figure 1: Strategy for nephron regeneration. MSCs were injected at the site of sprouting of the ureteric bud of E11.5 rat embryo (a). Theseembryos were developed in the whole-embryo culture system (b) for 48 hours. Embryos developed in this system shows the continuation ofdevelopment in this system ((c): arrow, yolk sac; asterisk, placenta).

Neokidney Original kidney

Desmin+ mesangium cells

Synaptopodin+ podocytes

Figure 2: Development of neokidney derived from hMSCs. Kidney anlagen derived from MSCs was transplanted into rat omentum. Afterthis relay culturing, hMSCs formed into neokidney in the rat omentum. Histological analysis revealed a well-differentiated kidney structure(right panels). Desmin-positive mesangial cells and synaptopodin-positive podocytes can be seen (right panels).

system of the recipient; therefore, the primary systemmust be modified to allow for vascular integration fromthe recipient to form a functional nephron. We used thefindings of Rogers et al. [8] described above, according towhich the metanephros can grow and differentiate into afunctional renal unit with integration of recipient vesselsif it is implanted into the omentum. We confirmed thatthe omentum is a suitable site for vascular integrationby comparing several transplantation sites [45]. We trans-planted metanephroi from different embryonic stages intothe omentum and found after 2 weeks that only metanephroifrom rat embryos older than embryonic day 13.5 developedsuccessfully. Therefore, the relay culture system was modifiedsuch that organ culture was terminated within 24 hours,

by which time the metanephros was allowed to developsufficiently and the kidney primordia could be transplantedinto the omentum (termed “modified relay culture system”).As a result, an hMSC-derived “neo-” kidney was generatedthat was equivalent to a human nephron (Figure 2) [46].

To examine the origin of the vasculature in the neokid-ney, we generated LacZ-transgenic rats [47] as recipientsso that donor- and recipient-derived tissues would be dis-tinguishable by X-gal assay. The usefulness of geneticallymarked transgenic (tg) rats in organogenesis has beenconfirmed previously, using GFP as a marker, in the rat.Thus, we chose to use the LacZ tg rat, which expressesthe marker gene ubiquitously, to determine the vascularorigin in our experiments [47]. Using the modified relay


Metanephric mesenchyme(MM) region

Transplantation

Differentiation

hMSCs

Nephron neogenesis Collecting duct system neogenesis

hMSCs

Chicken WD/UBprogenitor region

Transplantation

Transfection

PAX2

Differentiation

WDs

Collecting duct system

UBs

Nephrons

Whole kidney

GDNF

Transfection

Figure 3: Dual step for the development of whole functional kidney.

Removal of bone marrow cells

Establishment of renal stem cell

Culturing in growingxenoembryo

Transplantation

Reset life

iPS cell

Oxidative kidney injury

Figure 4: Putative scenario for the application of stem cell biology to kidney regeneration. Renal stem cells derived from bone marrow cellsor skin fibroblasts of a dialysis patient are cultured in a growing xenoembryo for a given time to develop into kidney primordia, followedby their autologous implantation into the omentum of the same patient. Kidney primordia eventually become a self-organ that performsmature renal functions. The patient may experience relief from dialysis and completely overcome the renal disease.

culture, we found that several vessels from the omentumappeared to be integrated into the neokidney, and X-galstaining showed that most of the peritubular capillarieswere LacZ positive, suggesting that they were of recipientorigin. Furthermore, electron microscope analysis revealedred blood cells in the glomerular vasculature. These dataindicated that the vasculature of the neokidney in theomentum originated from the host and communicated withthe host circulation, suggesting its viability for collectingand filtering the host blood to produce urine. To verifythis, we left the neokidney in the omentum for 4 weeksto develop further. What is surprising is that the structure

developed hydronephrosis, confirming the ability of theneokidney to produce urine. If the ureters were buriedunder the fat of the omentum, the urine would have noegress, resulting in hydronephrosis. Analysis of the liquidfrom the expanded ureter showed higher concentrations ofurea nitrogen and creatinine than the recipient sera in thenative urine. Therefore, we concluded that the neokidneythat developed in the omentum was capable of producingurine by filtering the recipient’s blood.

Production of erythropoietin (EPO) to maintain ery-thropoiesis is another important function of the kidney.EPO stimulates red blood cell production and is produced


mainly in the kidneys. We found 3 major findings regardingthe hMSC-derived neokidney in rats: (i) human EPO isproduced in rats harboring an organoid derived from autolo-gous human bone marrow cells, (ii) human EPO productionis stimulated by the induction of anemia, suggesting that thissystem preserves the normal physiological regulation of EPOlevels, and (iii) levels of EPO generated by the neokidneyin response to anemia in rats in which native EPO wassuppressed are sufficient to restore red cell recovery to a ratesimilar to that in control rats [48]. Taken together, these datasuggest that the neokidney derived from hMSCs may be ableto fulfill all renal functions, including urine production. Thiswork could lead to a new generation of therapy modalitiesfor kidney disease.

As mentioned earlier, kidney is formed by a reciprocalinteraction between the metanephric mesenchyme and theureteric buds and is, thus, derived from a collecting system(such as the ureters and the collecting ducts), which isa derivative of the ureteric bud, and other components(such as the glomeruli and tubules, which are derivativesof the metanephric mesenchyme). Our current system mayregenerate the derivatives of the metanephric mesenchymebut not the derivatives of the ureteric bud. Concerning thisissue, we assessed whether MSCs are able to differentiateinto the ureteric bud progenitor; we did this using chickembryos, which can be more easily manipulated and culturedas compared to mammalian embryos [49]. When hMSCsexpressing Pax2 were transplanted into the chicken uretericbud progenitor region, they migrated caudally with theelongating Wolffian duct and were integrated into theWolffian duct epithelia and then expressed LIM1, showingthat they can differentiate into the Wolffian duct cells underthe influence of local xenosignals [49]. This suggests thatthe whole kidney may be rebuilt by transplanting hMSCs atthe suitable time and part for derivatives of the metanephricmesenchyme and ureteric bud (Figure 3).

Based on our successes, we are currently investigatingthe possibility of experimenting on a larger animal, that is,the pig, because the porcine kidney has almost the samevolume as the human kidney. The ultimate size of thedeveloped metanephros appears to be imprinted during thevery early stages of development in the host embryo, asthe metanephroi of larger animals transplanted into theomenta of smaller hosts develop into organs of larger volume(diameter and weight) as compared to that of a normalhost kidney [50]. We hope that this system will facilitate thedevelopment of larger organs that are more suitable for usein humans (Figure 4).

6. Conclusion

In this article, we reviewed recent research on using renalstem cells to treat kidney diseases and proposed their possibleapplication in kidney regeneration. We know that prior to thetotal loss of renal structure, kidney function can be restoredby reactivating quiescent renal stem cells and/or supplyingrenal stem cells expanded sufficiently in vitro. Such stem cells

may contribute to kidney regeneration, leading to recoveryfrom organ failure.

We believe that emerging knowledge of kidney stem cellbiology and developmental biology will enable the develop-ment of new therapeutic strategies for kidney regenerationthat aim to regain damaged components of the kidney orrestore kidney function.

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